Hanrahan, a 31-year-old right-hander, saved 36 games for the Pirates last season and 40 in 2011, the first year in which he served as a full-time closer. With a fastball that reaches the upper-90s, he struck out more than a batter per inning in 2012.

The Red Sox could tab Hanrahan as their closer in 2013 and shift Andrew Bailey into a set-up role in hopes of solidifying their bullpen, but it is unclear what the Pirates need in exchange to complete the deal, according to Edes.

Hanrahan saved a total of 76 games over the last two seasons for the Pirates and is a year away from free agency. With incumbent closer Andrew Bailey still a question mark after a season in which he was injured for most of the year, then pitched ineffectively, it makes sense that the Sox would explore bullpen upgrades.

It is unclear what the Pirates would be getting back in return. Pittsburgh has been looking for pitching, primarily starters, but just came to terms with Francisco Liriano.

One major league source said Pittsburgh is seeking a "significant return" for Hanrahan, an All-Star in each of the last two seasons. He earned $4.1 million in 2012 and can expect a significant bump in salary arbitration, to close to $7 million.

CBSSports.com's Jon Heyman reports that the deal is likely for multiple players.